1. Field of the Invention
The present invention relates to a method of manufacturing a solid electrolytic capacitor using a conductive polymer such as a polypyrrole-based polymer or polythiophene-based polymer as a supporting electrolyte.
2. Description of the Prior Art
A well-known solid electrolytic capacitor is made of an anode consisting of a valve action metal such as aluminum or tantalum and having a dielectric oxide film formed on its surface and a cathode using a conductive polymer such as a polypyrrole-based polymer or polythiophene-based polymer as a supporting electrolyte.
A solid capacitor using tantalum for an anode is compact and has a large electrostatic capacitance. The characteristics of this capacitor are stable in a wide range from a low temperature to a high temperature. In addition, the capacitor has little voltage dependence and suffers little leakage current.
A conventional method of manufacturing a solid electrolytic capacitor in a case wherein an anode is formed by using tantalum will be described below.
(1) A molded member having a tantalum anode lead buried in tantalum metal powder by press molding is formed, and a porous tantalum element for a solid electrolytic capacitor is obtained by vacuum sintering.
(2) The tantalum element obtained in (1) is dipped in an aqueous phosphoric acid solution (0.6 wt %), and forming is performed by applying a voltage with the tantalum anode lead serving as a positive electrode and the electrode in the aqueous solution serving as a negative electrode.
(3) An insulating (dielectric) oxide film Ta.sub.2 O.sub.5 is formed on the surface of the tantalum element by forming.
(4) The tantalum element on which the oxide film is formed is dipped in 30 w % of an aqueous iron p-toluenesulfonate solution serving as an oxidant for 5 min. The resultant structure is dried and dipped in ethylenedioxythiophene monomer as a polymerization agent for 1 min. The resultant structure is dried to form a conductive polymer precoat layer inside the porous body of the tantalum element, thereby forming a primary chemical polymer layer.
(5) The tantalum element is dipped in an electrolytic polymerization solution using 0.7 mol/l of pyrrole as a monomer, and a power supply terminal is brought near to the tantalum element having undergone the above chemical polymerization. A conductive polymer layer is then formed on the surface of the element by electrolytic polymerization, thereby forming a secondary electrolytic polymer layer. In this case, the polymerization voltage is set to a standard electrode potential, i.e., 1 V, and the energization time is 20 min.
(6) After the completion of the above electrolytic polymerization, graphite and silver layers are formed on the surface of the tantalum element by paste coating and curing, and a cathode lead is formed on the resultant structure.
(7) The resultant element is packaged by resin molding or the like, thus obtaining a tantalum solid electrolytic capacitor.
As methods of improving the solid electrolytic capacitor obtained by the above manufacturing method, a solid electrolytic capacitor manufacturing method and a solid electrolytic capacitor and manufacturing method therefor are respectively disclosed in Japanese Unexamined Patent Publication Nos. 6-12086 and 3-64014.
According to the solid electrolytic capacitor disclosed in Japanese Unexamined Patent Publication No. 6-12086, in step (5) in which the secondary electrolytic polymer layer is formed, a surfactant is added to the electrolytic polymerization solution to obtain a solid electrolytic capacitor having a small dielectric tangent value and a low equivalent series resistance in a high-frequency range.
According to the solid electrolytic capacitor and manufacturing method therefor disclosed in Japanese Unexamined Patent Publication No. 3-64014, in step (5) described above, the electrostatic capacitance, Tan .delta., and the like are improved by adding a surfactant such as an anionic surfactant to the electrolytic polymerization solution for forming the secondary electrolytic polymer layer.
In the solid electrolytic capacitor manufacturing method disclosed in Japanese Unexamined Patent Publication No. 6-12086 and the solid electrolytic capacitor and manufacturing method therefor disclosed in Japanese Unexamined Patent Publication No. 3-64014, it was very difficult to select a surfactant to be added and adjust the concentration of the selected surfactant. When a secondary electrolytic polymer layer is formed by using a supporting electrolyte having a surface active effect, if the concentration of the surfactant added is high, the secondary electrolytic polymer layer peels off, and the electrostatic capacitance of the capacitor decreases. When a secondary electrolytic polymer layer is formed by using a supporting electrolyte having no surface active effect, many projections are formed on the conductive polymer layer, resulting in a poor shape.
FIG. 1 is a graph showing the electrostatic capacitances of capacitors each obtained by adding only one type of surfactant to the electrolytic polymerization solution in step (5) described above. FIG. 2 shows the outer appearance of each secondary electrolytic polymer layer formed by adding one type of surfactant to the electrolytic polymerization solution.
(Prior Art 1): When only sodium dodecylbenzenesulfonate (to be abbreviated to Na-DBS hereinafter) having a surface active effect is added as a supporting electrolyte to the electrolytic polymerization solution, polymerization can be uniformly performed. However, the electrostatic capacitance (1 kHz), which should be about 160 .mu.F, becomes 143 .mu.F. That is, the electrostatic capacitance decreases. In this case, the outer appearance does not change.
(Prior Art 2): When only sodium p-toluenesulfonate (to be abbreviated to Na-pTS hereinafter) having no surface active effect is added as a supporting electrolyte to the electrolytic polymerization solution, nonuniform projections are formed on the electrolytic polymer layer. As a result, the capacitor has a poor outer appearance. In this case, the electrostatic capacitance (1 kHz) is 165 .mu.F, and hence does not decrease.
(Prior Art 3): When only sodium butylnaphthalenesulfonate (to be abbreviated to Na-BNS hereinafter) having a surface active effect is added as a supporting electrolyte to the electrolytic polymerization solution, the electrostatic capacitance (1 kHz), which should be about 160 .mu.F, becomes 141 .mu.F. That is, the electrostatic capacitance decreases. In this case, the outer appearance does not change.
(Prior Art 4): When only sodium naphthalenesulfonate (to be abbreviated to Na-NS hereinafter) having a surface active effect is added as a supporting electrolyte to the electrolytic polymerization solution, nonuniform projections are formed. As a result, the capacitor has a poor outer appearance. The electrostatic capacitance (1 kHz) is 164 .mu.F, and hence does not decrease.